Worm gear drive

ABSTRACT

A worm gear drive in which a worm coil is curved along the periphery of a worm wheel and has convolutions formed with peripheral grooves which receive rolling bodies which mesh with teeth at the periphery of the worm wheel.

United States Patent Titt [s41 WORM GEAR DRIVE Georg Tltt, D-8358 ObereVorstadt 9,

[4 1 June 27, 1972 3,377,879 4/1968 Kozo Shiwaku 74/425 3,468,179 9/ l969 Sedgwick et a1.

3,489,026 1/1970 Bond 3 ,494,215 2/1970 Fengler 741459 X 3,581,5926/1971 Roehrs et a] ..74/464 Primary Examiner-Leonard H. GerinAttorney-Steinberg and Blake ABSTRACT A worm gear drive in which a wormcoil is curved along the periphery of a worm wheel and has convolutionsfonned with peripheral grooves which receive rolling bodies which meshwith teeth at the periphery of the worm wheel.

10 Claims, 4 Drawing Figures PATENT'EDJum I972 3, 672 239 sum 2 or 2 INV EN TOR riorzc T/TT M M i s my WORM GEAR DRIVE BACKGROUND OF THEINVENTION The present invention relates to worm gear drives.

With conventional worm gears, the structure almost invariably takes theform of a hindleys or hourglass screw so that a plurality of wormconvolutions will participate in the transmission. The manufacture ofsuch worms with corresponding rolling paths for rolling bodies isdifficult to carry out, and the same is true of a worm wheel whichcooperates with such a worm. Above all the grinding of the rolling pathafter hardening of the metal creates problems. However, not only hasthis latter disadvantage prevented practical utility of drives of thistype up to the present time, but in addition the operating conditionsencountered are not ideal. The hourglass screw does not operate with aconstant pitch, and the rolling bodies travel at different speeds, whilethe configuration of the worm screw convolutions change incorrespondence with the rolling action, so that the rolling bodiesencounter different meshing conditions. The final effect is that onlypart of the rolling bodies can be expected to carry the transmissionload.

SUMMARY OF THE INVENTION It is accordingly, a primary object of thepresent invention to provide a worm gear drive where the power istransmitted between the worm and worm wheel through balls or otherrolling bodies, while avoiding the above drawbacks encountered withconventional worm gear drives.

In particular, it is an object of the present invention to provide aworm gear drive of this type which can achieve an efficiency which isfar greater than that encountered with conventional worm gear drives.

Also, it is an object of the present invention to provide a worm geardrive of this type which can have an extremely large transmission ratio.

A further object of the present invention is to provide a worm geardrive of the above type which canreadily be manufactured at relativelylow cost while avoiding the difficulties encountered in the manufactureof conventional worm gear drives.

Yet another object of the present invention is to provide a worm geardrive where a considerable load can be transmitted while at the sametime any individual part of the structure of the invention is not verygreatly loaded.

A further object of the invention is to provide a construction of thistype which is simple and rugged so that all of the advantages of theinvention can be achieved at a relatively low cost and with a structurewhich will operate reliably over a long period of time.

According to the invention the worm gear drive includes a rotary wormwheel having a periphery provided with teeth, and a plurality of rollingbodies which engage these teeth. A worm coil is curved along theperiphery of the worm wheel and has convolutions respectively providedwith grooves which receive the rolling bodies and define a path oftravel therefor.

BRIEF DESCRIPTION OF DRAWINGS The invention is illustrated by way ofexample in the accompanying drawings which form part of this applicationand in which:

FIG. I is a fragmentary partly sectional elevation taken in a planenormal to the axis of the wonn wheel and illustrating one possibleembodiment of a worm gear drive according to the invention;

FIG. 2 is a fragmentary sectional elevation in the same plane as FIG. 1showing the cross section of a convolution of the worm coil and alsoillustrating the manner in which the worm coil coacts with the wormwheel through a rotary body;

FIG. 3 shows a structure similar to FIG. 2 where the convolution of theworm coil has a greater elasticity; and

FIG. 4 is a schematic representation of a synchronizing assembly to beoperatively connected with the worm coil for synchronizing the rotationof opposed ends thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS through the angle )8 to thecurvature of the periphery of the worm wheel 1. The convolutions of theworm coil 2 are respectively formed with peripheral grooves whichreceive the rolling bodies 6 and form a path of travel therefor.

As is apparent particularly from FIGS. 2 and 3, the convolutions of theworm coil 2 are of a tapered cross section atleast in the region of theconvolutions formed with the peripheral grooves which receive therolling bodies 6. As a result the convolutions of the worm coil at leastat the part thereof extending through the angle )3 are capable ofengaging each other at their side surfaces almost throughout the entirewidth of these side surfaces. The direct engagement between these sidesurfaces of the convolutions which are at the smallest radius ofcurvature of the worm coil along the angle p is illustrated in FIG. 1.

Beyond this latter angle the worm coil 2 has some convolutions, in theregion of the opposed ends of the worm coil, which no longer follow thecurvature of the worm wheel 1 but which instead are positionedtangentially with respect to the periphery of the worm wheel 1. Shafts 3and 4 are respectively fixed to the opposed ends of the worm coil 2.

These shafts 3 and 4 are supported for rotary movement in anunillustrated housing of the worm gear drive. This housing carriessuitable thrust bearings which maintain the shafts 3 and 4 inaxiallypredetermined positions while at the same time supporting them forrotary movement. These shafts 3 and 4 respectively terminate in enlargedend portions 5 on which the end convolutions of the coil 2 are placed soas to be centered by these end portions 5. The shafts 3 and 4 arerespectively fixed with flanges 23 to which the end convolutions of thecoil are respectively fixed so that through these flanges 23 the axialpositions of the end convolutions are determined.

The teeth at the periphery of the worm wheel I are in the form of ahollow circular channel which receives the coil 2 and which extendscircumferentially around the latter approximately up to the centerthereof, so that the periphery ofthe worm wheel extends approximatelyhalf way around the worm coil 2. The remainder of the worm coil issurrounded by a shell 11 made of'a hardened steel and having an innerground surface with which the rolling bodies 6 make rolling contact whenthey travel beyond the periphery of the worm wheel 1. In this way therolling bodies 6 serve to absorb the radial forces at the worm. Thisshell 11 is also fixed to the unillustrated housing of the worm geardrive.

A pair of end tubes 7, in which the rolling bodies 6 can travel, areoperatively connected with a pair of convolutions of the coil 2,respectively, situated at opposed end regions of the latter, and theseend tubes 7 are interconnected by an elastic connecting tube 8 whichextends along the interior of the coil, so that the end tubes 7 and theconnecting tube 8 form together with the peripheral grooves of theconvolutions of the coil 2 an endless path of travel for the rollingbodies 6 which roll at approximately half the circumferential speed ofthe worm. Thus through the tubes 7 and 8 it is possible for the balls 6to travel from one end region of the worm coil 2 through the interior ofthe latter to the other end region thereof. The end tubes 7 respectivelyterminate in fingers 10 which do not extend beyond the path of therolling bodies 6 and serve to guide the balls 6 smoothly into one of theend tubes 7 and out of the other of the end tubes 7. The fingers l0guide balls 6 smoothly with respect to the end tubes 7 which communicatewith the grooves of the convolutions to which they are respectivelyfixed at depths somewhat deeper than these grooves. It is preferred toform the fingers 10 as integral parts of the end tubes 7. It will benoted that these end tubes 7 do not project outwardly beyond theexterior surface of the worm coil 2 which projects beyond the entire endtubes 7.

These tubes 7 in addition to being fixed respectively to a pair ofconvolutions at the end regions of the coil 2 are fixed to the flanges23 through which these tubes 7 extend, and beyond these flanges thetubes 7 are connected to and communicate with the elastic connectingtube 8. The latter tube extends along the interior of the coil so as toconvey the rolling bodies through the worm from one end tube 7 to theother. The connection 9 between each end tube 7 and an end of the tube 8can take the form of a simple plug-and-socket type of connectionaccording to which the ends of the tube 8 are simply pressed into endsof the tubes 7, respectively. After this latter connection of the tube 8with the tubes 7 is formed, one end of the tube 8 can be fixed with theshaft 3 and the other end can be fixed with the shaft 4 through suitableclamping connections, while the end tubes 7 themselves mayadvantageously be fixed in position with a hard solder. The flexibleconnecting tube 8 can be made of plastic or it can take the form of awire coil made of a wire of square cross section.

FIG. 2 illustrates the tapered, wedge-shaped profile of the convolutionsof the worm 2. FIG. 2 also illustrates the manner in which the balls 6mesh with the worm wheel and worm. The

peripheral groove formed in each convolution of the coil 2 hassymmetrical side surfaces 21 the configuration of which is designed toprovide a rolling path for the rolling bodies where the meshing angle isas small as possible, while at the same time maintaining a pressurediagram which does not extend beyond the edges of the side surfaces 21.With this meshing angle a shown at the right in FIG. 3, the transmissionof the invention will when loaded transfer the force from the worm 2 tothe worm wheel 1. Because of the elasticity present in the system, theworm is displaced slightly away from the worm wheel, so that the rollingbodies only engage the path of travel provided therefor at the loadpoints. However, when the transmission idles or runs without anyloading, the rolling bodies 6 can fall back to the base surfaces 22 ofthe grooves which define the path of travel for the rolling bodies 6, sothat at this time the rolling bodies 6 roll along the base surfaces 22of the grooves. In this way direct engagement between the worm 2 and theworm wheel 1 is avoided.

In the embodiment of FIG. 3 the convolutions of the worm coil are eachprovided with a cross section formed at a peripheral portion opposite tothe peripheral portion provided with the groove which receives therolling bodies 6 with a second peripheral groove 12, so that in this waythe elasticity of each convolution is increased and each convolution canyield elastically to compression forces. For this reason it is possibleto provide for the embodiment of FIG. 3, at each of the convolutions,side surfaces 14 which are inclined with respect to each other at asomewhat larger angle of inclination than in the case of FIG. 2, so thatonly after curving of the worm along the periphery of the worm wheelwill the convolutions engage each other, at the angular region B alongsubstantially the entire width of the side surfaces of the convolutions.Moreover, with the embodiment of FIG. 3 the circumferential length ofengagement of the convolutions at the regions next to the periphery ofthe worm wheel and at the angular region B is greater than with theconvolution cross section which is illustrated in FIG. 2.

The inclined side surfaces I4 of each convolution extend from relativelynarrow peripheral side surface regions 13 which are substantiallyperpendicular to the axis of the worm when the latter is in a straightcondition. The end convolutions which do not bear the load engage eachother at these regions 13.

In order to reliably maintain a synchronized rotary movement for theworm shafts 3 and 4, they may be interconnected by the synchronizingstructure which is shown in FIG. 4. This synchronizing structureincludes a pair of end synchronizing shafts l5 and 16 and anintermediate synchronizing shaft 17 axially situated between the endsynchronizing shafts l5 and 16. A universal joint interconnects each ofthe end shafts I5 and 16 with the intermediate shaft 17. Each universaljoint includes the forked or bifurcated members 18 and 19 fixed to anelastic ring 20. Each of the members 18 and 19 may take the form, forexample, of a pair of fingers extending in opposite lateral directionsfrom the axis of the universal joint, the pair of fingers of member 18being displaced by with respect to the pair of fingers of member 19.Thus, the ends of the fingers of member 18 will be fixed to the elasticring 20 at diammetrically opposed parts of the latter while the ends ofthe fingers of member 19 will be fixed to the ring 20 also atdiammetrically opposed parts of the latter, with these connections ofthe fingers of member 19 being displaced by 90 with respect to theconnections of the fingers of member 18. These members 18 and 19respectively have tubular hub portions which are respectively fixed withthe shafts 15-17 in the manner shown in FIG. 3. The shafts l5 and 16 mayextend parallel to the shafts 3 and 4 and gears which are fixed to theshafts 15 and 16 may mesh with gears which are fixed to the shafts 3 and4, so that through the structure of FIG. 4 the shafts 3 and 4 will becompelled to rotate in synchronism with each other.

With the above-described structure of the invention the disadvantages ofconventional structures are avoided and at the same time severaladditional advantages are achieved. Thus, with the invention a curvedflexible worm is used instead of a straight massive worm. When the wormof the invention is in a straight condition it has the configuration ofa spiral or coil spring. By curving this worm of the invention it ispossible to adapt it to a predetermined peripheral portion of the wormwheel. In this way great advantages are achieved in that eachconvolution of the worm coil is situated in a radial plane whichcontains the axis of the worm wheel so that each convolution has withrespect to the worm wheel an operating relation which is as good as thatwhich could be achieved if the worm gear were provided with a wormhaving only a single convolution.

Moreover, when the worm of the invention is manufactured this worm canbe in a straight condition so that it is simple to carry out thegrinding operations. The elongated bar or rod stock from which the wormcoil is manufactured can initially be advantageously drawn so that thesoft working thereof can be very simply and effectively carried out.Under suitable conditions further operations on the worm are notrequired after the winding of the convolutions thereof and the hardeningthereof.

As a result of curving of the worm along the periphery of the wormwheel, the individual convolutions press against each other at the innerpart of the worm which directly engages the worm wheel, so that as aresult of the frictional connection between these convolutions therequired circumferential force is transmitted from one convolution tothe next. It is for this purpose that the present invention provides forthe convolutions the tapered cross section as shown in FIGS. 2 and 3 anddescribed above, so that in this way, in correspondence with thediameter of the worm wheel, the individual convolutions can engage eachother through as great a width as possible in order to reduce thepressure per unit of area to as low a value as possible. With the groove12 shown in FIG. 3 it is possible to provide each convolution of theworm with an elasticity at its inner region which will achieve also inthe circumferential direction a longer length of engagement between theindividual convolutions. In this way it is possible to select a suitablecompromise according to which in accordance with the given direction ofrotation of the convolutions the entire thrust of the load isdistributed among all of the convolutions without overloading thelatter, while on the other hand as large a number of convolutions aspossible participate in the meshing of the rolling bodies with the wormwheel.

A further advantage of the invention resides in the fact that the wormwheel can circumferentially extend around the worm through orapproximately half way around the worm, without creating in this way anydifferent operating conditions. Inasmuch as the individual wormconvolutions are elastic through this region, the pressure changes fromthe central or deepest part of the worm wheel periphery where the wormconvolutions directly engage each other to the outer edges of the wormwheel channel which receives the worm. Bending of the worm coil to acondition where it will assume a larger curvature then that which it haswhen cooperating properly with the worm wheel cannot take place since inthis event the individual convolutions of the worm will come to pressthrough the rolling bodies against the unloaded sides of the path oftravel provided for the rolling bodies in the worm wheel.

The shell 11 referred to above forms a further feature of the inventionsince this shell also serves to take up the radial forces of the wormwhile at the same time providing a surface of rolling contact for therolling bodies when they travel 'beyond the worm wheel. Withconventional worm gears the bearings of the worm are required to absorbthese radial forces. Theseadvantages' are achieved with the feature ofthe invention according to which the rolling path for the rollingbodies, both at the worm and the worm wheel, is made up of threesurfaces of curvature provided for the surfaces 21 and 22. Thesymmetrical surfaces 21 have the same curvature which permits thesmallest possible angle of engagement a, while the base 22 remains toengage the rolling bodies so as to prevent any direct engagement betweenthe worm and worm wheel as soon as a load is no longer transmitted, sothat the worm and worm wheel can never come-into direct engagement witheach other.

With the feature of the invention according to which some of the endconvolutions at each of the end regions of the worm are positionedtangentiallywith respect to the worm wheel, these end convolutionsdirectly engage each other along their entire lengths. The shafts 3 and4 which are connected to the ends of the worm coil are themselvessupported for rotary movement in suitable bearings within the housing ofthe worm gear drive. The removal of rolling bodies from the worm and thereturn of the rolling bodies to the worm takes place at,

these end regions of the worm, so that the steel end tubes 7 with theirfingers 10 are fixed only to the free end regions of the worm and to theshafts 3 and 4, while the bendable connecting tube 8 is situated in theinterior flexible portion of the worm in order to provide the endlesspath of travel for the balls 6. As is pointed out above the connectingtube 8 can be formed from a coil spring the convolutions of whichdirectly engage each other and the wire of which has a square crosssection.

The length of the curved worm can have any desired magnitude, while thediameter is chosen in such a way that the profile or cross section ofeach convolution is deformed only to a small extent. A relatively smallfriction factor of approximately 0.001 to 0.0015 for the rollingfriction enables an extremely small pitch to be provided for the worm ofthe invention without any appreciable losses.

As is pointed out above in connection with FIG. 4, it is also a featureof the invention to provide for synchronized rotary movement of the endsof the worm by way of the synchronizing shafts -17 and the universaljoints therebetween.

What is claimed is:

1. In a worm gear drive, a worm wheel having a periphery provided withteeth, rolling bodies engaging said teeth and a worm coil curved alongpart of the periphery of said worm wheel and having convolutionsrespectively formed with peripheral grooves receiving said rollingbodies and defining a path of travel therefor.

2. The combination of claim 1 and wherein said convolutions of said wormcoil are of a tapered cross section at least in the regions thereofprovided with said grooves, and said convolutions having side surfacesdirectly engaging each other in the immediate vicinity of said peripheryof said worm wheel.

3. The combination of claim 1 and wherein said convolutions arerespectively formed with peripheral grooves opposed to those whichreceive said rolling bodies for increasing the elasticity of saidconvolutions.

4. The combination of claim 1 and wherein said worm coil has opposed endregions provided with convolutions positioned tangentially with respectto said periphery of said worm wheel.

5. The combination of claim 1 and wherein said periphery and a hollowshell extending around the remainder of said worm coil and having aninner surface engaged by said rolling bodies when they travel beyondsaid teeth of said worm wheel.

6. The combination of claim I and wherein a pair of end tubes in whichsaid rolling bodies travel are respectively fixed to a pair of saidconvolutions at opposed end regions of said worm coil, and a connectingtube in which said rolling bodies also travel extending between andinterconnecting said end tubes, said end tubes and connecting tubeforming with the peripheral grooves of said convolutions an endless pathof travel for said rolling bodies.

7. The combination of claim 6 and wherein said end tubes respectivelyhave fingers engaging the rolling bodies at the peripheral grooves ofsaid convolutions to which said end tubes are respectively fixed, saidfingers respectively guiding said rolling bodies into one of said endtubes from the peripheral groove of the convolution to which the latterend tube is fixed and out of the other end tube into the peripheralgroove of the convolution to which said other end tube is fixed.

8. The combination of claim 7 and wherein said worm coil has opposed endregions each provided with convolutions positioned tangentially withrespect to said periphery of said worm wheel, a pair of flangesrespectively fixed to opposed ends of said worm coil, and a pair ofshafts respectively fixed to said flanges, said convolutions to whichsaid end tubes are respectively fixed forming a pair of those which arepositioned tangentially with respect to said periphery of said wormwheel and said end tubes also being fixed with said flanges.

9. The combination of claim 1 and wherein said peripheral groove of eachconvolution is defined by a pair of opposed symmetrical side surfacesand a base surface extending between said side surfaces.

10. The combination of claim I and wherein a pair of worm shafts arerespectively connected with opposed ends of said worm coil, and a seriesof three synchronizing shafts axially spaced from each other andextending along the exterior of said worm coil and worm shafts, saidsynchronizing shafts including a pair of end shafts and an intermediateshaft situated therebetween, and a universal joint situated between andoperatively connected to each of said end shafts and said intermediateshaft, so that by connecting said end synchronizing shafts to said wormshafts the latter can be compelled to rotate in synchronism with eachother.

1. In a worm gear drive, a worm wheel having a periphery provided withteeth, rolling bodies engaging said teeth and a worm coil curved alongpart of the periphery of said worm wheel and having convolutionsrespectively formed with peripheral grooves receiving said rollingbodies and defining a path of travel therefor.
 2. The combination ofclaim 1 and wherein said convolutions of said worm coil are of a taperedcross section at least in the regions thereof provided with saidgrooves, and said convolutions having side surfaces directly engagingeach other in the immediate vicinity of said periphery of said wormwheel.
 3. The combination of claim 1 and wherein said convolutions arerespectively formed with peripheral grooves opposed to those whichreceive said rolling bodies for increasing the elasticity of saidconvolutions.
 4. The combination of claim 1 and wherein said worm coilhas opposed end regions provided with convolutions positionedtangentially with respect to said periphery of said worm wheel.
 5. Thecombination of claim 1 and wherein said periphery of said worm wheel hasthe configuration of a hollow channel and extends approximately half wayaround said worm coil, and a hollow shell extending around the remainderof said worm coil and having an inner surface engaged by said rollingbodies when they travel beyond said teeth of said worm wheel.
 6. Thecombination of claim 1 and wherein a pair of end tubes in which saidrolling bodies travel are respectively fixed to a pair of saidconvolutions at opposed end regions of said worm coil, and a connectingtube in which said rolling bodies also travel extending between andinterconnecting said end tubes, said end tubes and connecting tubeforming with the peripheral grooves of said convolutions an endless pathof travel for said rolling bodies.
 7. The combination of claim 6 andwherein said end tubes respectively have fingers engaging the rollingbodies at the peripheral grooves of said convolutions to which said endtubes are respectively fixed, said fingers respectively guiding saidrolling bodies into one of said end tubes from the peripheral groove ofthe convolution to which the latter end tube is fixed and out of theother end tube into the peripheral groove of the convolution to whichsaid other end tube is fixed.
 8. The combination of claim 7 and whereinsaid worm coil has opposed end regions each provided with convolutionspositioned tangentially with respect to said periphery of said wormwheel, a pair of flanges respectively fixed to opposed ends of said wormcoil, and a pair of shafts respectively fixed to said flanges, saidconvolutions to which said end tubes are respectively fixed forming apair of those which are positioned tangentially with respect to saidperiphery of said worm wheel and said end tubes also being fixed withsaid flanges.
 9. The combination of claim 1 and wherein said peripheralgroove of each convolution is defined by a pair of opposed symmetricalside surfaces and a base surface extending between said side surfaces.10. The combination of claim 1 and wherein a pair of worm shafts arerespectively connected with opposed ends of said Worm coil, and a seriesof three synchronizing shafts axially spaced from each other andextending along the exterior of said worm coil and worm shafts, saidsynchronizing shafts including a pair of end shafts and an intermediateshaft situated therebetween, and a universal joint situated between andoperatively connected to each of said end shafts and said intermediateshaft, so that by connecting said end synchronizing shafts to said wormshafts the latter can be compelled to rotate in synchronism with eachother.